why are the kills always off screen and characters are mostly not given names.

These are just guesses on my part.

1 - Probably money. Effects take coin, and those SOV serial killer movies have zero budget.
2 - Probably to not get sued. If the kills are actually based on actual events then the families of the deceased are in a position to sue for infringing upon their story rights. There comes a point when the murderer becomes such a high profile celebrity that his rights become public domain. The same doesn't happen for the deceased. I believe you can get past this by simply not naming the characters, or simply changing their name.

>>2
thank you for a detailed answer

Why the hell is this talentless hack still making movies?
Does this moron have a fanbase?